1. Field of the Invention:
The present invention generally relates to a liquid crystal display device and, more particularly, to a twisted nematic (TN) liquid crystal display device.
2. Background of the Invention:
Liquid crystal display devices are utilized in numerous applications, for example, clocks, wrist watches, electronic calculators, computer terminal devices, word-processor display devices, compact television receiver sets, and so on.
A supertwisted birefringence effect (SBE) display mode in which liquid crystal molecules are twisted to about 180.degree. to about 270.degree. for the purpose of improving the contrast and which is capable of exhibiting an improved multiplex driving characteristic is currently widely used in liquid crystal devices. An example of the liquid crystal display cell using the SBE display mode is schematically illustrated in FIG. 5 of the accompanying drawings, reference to which will now be made.
In FIG. 5, the nematic liquid crystal having a positive dielectric anisotropy is disposed between a pair of electrode-formed substrates 17 and 18. The liquid crystal exhibits a helical structure having a twist angle .phi., and orientation is achieved by the use of, for example, a slant vapordeposition method or a rubbing method in which a surface of each of the electrode substrates which is in contact with the liquid crystal contacts is rubbed in one direction with a cloth. An optically active material is added to the nematic liquid crystal so that the ratio of the thickness d of the liquid crystal cell relative to the resultant helical pitch P, that is, d/P, can fall within the range expressed by the following equation. EQU (.phi./2.pi.-1/4)&lt;d/P&lt;(.phi./2.pi.+1/4)
The typical voltage-transmission characteristic curve of the SBE mode liquid crystal cell is illustrated in FIG. 6. The direction in which observation is made is a direction normal to the liquid crystal display surface. In FIG. 6, as a parameter descriptive of a steep characteristic of a threshold characteristic of the voltage-transmission, a value .alpha. is defined which stands for the ratio of the voltage V.sub.10 %, at which the transmission attains 10%, relative to the voltage V.sub.90 % at which the transmission attains 90%. As the value .alpha. approaches 1, the threshold characteristic becomes steep, and the multiplex driving characteristic improves. In view of this, where a SBE mode liquid crystal cell wherein phenylcyclohexanes are employed as the liquid crystal is used as compared to where a SBE mode liquid crystal cell having a 90.degree. twisted nematic liquid crystal is employed, it has been found that the value .alpha. exhibited by the SBE mode liquid crystal cell wherein phenylcyclohexanes are employed is 1.08 whereas the value .alpha. exhibited by the SBE mode liquid crystal cell wherein the conventional 90.degree. twisted nematic liquid crystal is employed is 1.40. In this way, it will readily be understood that the SBE mode liquid crystal cell has a very steep threshold characteristic.
FIG. 7 illustrates the dependency of the transmission on the wavelength when the SBE mode liquid crystal display cell is in an ON state and in an OFF state, and FIG. 8 illustrates the dependency of the transmission on the wavelength when the conventional 90.degree. twisted nematic liquid crystal cell is in an ON state and in an OFF state. As can be understood from the respective graphs shown in FIGS. 7 and 8, the dependency of the transmission of the SBE mode liquid display cell on the wavelength is more considerable than that of the twisted nematic liquid crystal cell. This is because the SBE display mode makes use of birefringent effects of the liquid crystal. Accordingly, in the SBE mode liquid crystal cell, it is not possible to accomplish a uniformly switching across the visible light sprectrum and a undesirable coloring therefore occurs in the display, thereby constituting a cause of reduction in display quality.